1. Field of the Invention
The present invention relates to a wire bonding method in which a first bonding point and a second bonding point are connected by a bonding wire in a semiconductor device manufacturing process and more specifically to a wire loop formation method employed in such a wire bonding method.
2. Prior Art
As shown in FIG. 4(a) and 4(b), in a semiconductor device, a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (second bonding point) on the lead frame 1 are connected by a bonding wire (called merely "wire") 3. The loop shape of the wire 3 in this case may be a trapezoidal loop shape as shown in FIG. 4(a) or a triangular loop shape as shown in FIG. 4(b).
Wire loop formation methods of this type are described in, for example, Japanese Patent Application Publication (Kokoku) No. 5-60657 and Japanese Patent Application Laid-Open (Kokai) No. 4-318943.
The trapezoidal loop shown in FIG. 4(a) is formed by the process shown in FIG. 5.
In step (a) in FIG. 5, a capillary 4 is lowered with a clamper (not shown) which holds, in its closed state, a bonding wire 3, so that a ball formed on the tip end of the wire is bonded to a first bonding point A. After this, the capillary 4 is raised to point B, delivering the wire 3.
Next, as seen in step (b), the capillary is moved horizontally in the opposite direction from the second bonding point G to point C. Generally, a loop formation operation in which the capillary 4 is moved in the direction opposite from the second bonding point G is referred to as a "reverse operation". Because of this reserve operation, the wire 3 assumes a shape that extends from point A to point C; and as a result, a kink 3a is formed in a portion of the wire 3. The wire 3 delivered out of the capillary 4 in the process from point A to point C forms the neck height portion 31 of the loop shown in FIG. 4(a).
Next, as shown in step (c) in FIG. 5, the capillary 4 is raised to point D, delivering the wire 3.
Afterward, as shown in step (d), the capillary 4 is again moved horizontally to point E in the opposite direction from the second bonding point G, i.e., another (or second) reverse operation is performed. As a result, the wire 3 assumes a shape inclined from point C to point E, and a kink b is formed in a portion of the wire 3 by the lower end portion of the capillary 4. The wire 3 delivered out of the capillary 4 during the process from point C to point E forms the trapezoidal length portion 32 shown in FIG. 4(a).
In addition, as shown in step (e), the capillary 4 is raised to point F delivering an amount of wire 3 equal to the inclined portion 33 shown in FIG. 4(a). Afterward, the clamper (again, not shown) is closed. Once the clamper is closed, the wire 3 is not delivered out even if the capillary 4 subsequently is moved.
Furthermore, as shown in steps (f) and (g), the capillary 4 performs a circular-arc motion (or a circular-arc motion followed by a straightly lowering motion) so that the capillary 4 is positioned at the second bonding point G, and the wire 3 is bonded to the second bonding point G, thus connecting the first and second bonding points A and G.
On the other hand, the triangular loop shown in FIG. 4(b) is formed by the process shown in FIG. 6.
In this triangular loop formation, the trapezoidal length portion 32 described in the above loop formation is not formed. Accordingly, the second reverse operation in step (d) in FIG. 5 is not performed. Thus, the steps (c), (d) and (e) in FIG. 5 are replaced by the single process as shown in step (c) of FIG. 6. In particular, the steps (a) and (b) are the same as the steps shown in FIG. 5, respectively; and after the first reverse operation in step (b) of FIG. 6, the capillary 4 is raised to point F delivering the wire 3 in step (c). Afterward, the capillary 4 performs the operations steps (d) and 6(e) in the same manner as the operations done in the steps (f) and (g) shown in FIG. 5, so that the wire 3 is bonded to the second bonding point G.
As seen from the above, the triangular loop formation shown in FIG. 6 is simpler than the trapezoidal loop formation shown in FIG. 5 and is therefore advantageous in that the loop formation is performed in a shorter time. However, in cases where the step height between the first bonding point A and the second bonding point G is large (for instance, about 100-350 .mu.m), or in cases where the first bonding point A and the edge portion of the semiconductor chip 2 are separated by a conspicuously large distance (for instance, about 500 .mu.m), the wire 3 tends to come into contact with the edge portion of the semiconductor chip 2 when the triangular wire loop shape as shown in FIG. 4(b) is formed. In such cases, the trapezoidal wire loop formation is employed to form the wire as shown in FIG. 4(a) so as to avoid the contact between the wire 3 and semiconductor chip 2.
In the trapezoidal loop formation shown in FIG. 5, whether or not the kink b can be formed in the wire 3 by the capillary 4 using the reverse operation depends upon the wire length (height) from the first bonding point A to the kink b. As the height increases, formation of the kink b tends to become more difficult. The first reverse operation shown in step (b) of FIG. 5 is performed with the capillary 4 positioned at a height near the height of the first bonding point A; accordingly, a relatively strong kink 3a can easily be formed.
However, the second reverse operation in step (d) in FIG. 5 is performed with the capillary 4 at a high position which is far from the first bonding point A; accordingly, the kink 3b is difficult to form, and its angle is unstable from kink to kink. As a result, since the portion of the wire near the kink 3b shown in FIG. 4(a) is unstable and has a weak shape-retaining strength, the portion of the wire near the kink 3b may rise upward to a point higher than the portion of the wire near the kink 3a. If the shape-retaining strength of the portion of the wire near the kink 3b is weaker than the portion of the wire near the kink 3a, then the wire bends when a pressure is applied thereon. For example, wire bending may easily occur by external forces such as shocks or vibration of the wire 3 because the capillary contacts the second bonding point G or because the ultrasonic oscillation is applied during bonding of the second bonding point G. Such a wire bending may be further caused by a mold flow due to the injection of molding material during molding, etc.
Accordingly, so as to avoid these problems, the shape-retaining strength of the kink 3b is reinforced in conventional methods by increasing the amount of reverse movement of the capillary in the step (d) shown in FIG. 5. In this case, however, since the capillary 4 needs to be moved a further distance, bonding takes longer because of the corresponding longer moving amount; and in addition, since the neck portion of the wire is bent further, separate problems, such as a neck damage, likely to occur.